The days when people thought only heavy Cheech-and-Chong pot smokers suffered cognitive consequences may be over. A study in The Journal of Neuroscience says even casual marijuana smokers showed significant abnormalities in two vital brain regions important in motivation and emotion.

“Some of these people only used marijuana to get high once or twice a week,” said co-author Hans Breiter, quoted in Northwestern University’s Science Newsline. Breiter hailed the study as the first to analyze the effects of light marijuana use. “People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school,” he said. “Our data directly says this is not the case.”

“This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” he added.

The study analyzed 20 pot smokers and 20 non-pot smokers between 18 and 25. Scientists asked them to estimate how much marijuana they smoked and how often they lit up over a three-month test period. Even those who smoked once a week showed brain abnormalities, while larger changes were seen in those who smoked more.

Marijuana is by far the most recognizable drug in the United States, with almost 19 million people reporting recent use, according to the National Survey on Drug Use. Cultural attitudes toward the drug are changing fast. What would have been inconceivable a generation ago — the decriminalization and legalization of marijuana — has happened in several states over the last several years. Nascent industries around the plant have sprouted in Colorado and Washington since they legalized the drug.

The study did not look at the behavior of the pot smokers, only their brains. What effect, if any, Wednesday’s findings will have on future legislation remains unclear.

The drug’s effect on the human brain, however, is substantially more clear, researchers say. In the study, scientists compared the size, shape and density of the nucleus accumbens and the amygdala, which control emotion. Those who had smoked had abnormally large nucleus accumbens, an area of the brain that controls pleasure, reward, and reinforcement learning.

In the brains of marijuana users, natural rewards are less satisfying.

“Drugs of abuse can cause more dopamine release than natural rewards like food, sex and social interaction,” said lead author Jodi Gilman. “In those you also get a burst of dopamine but not as much as in many drugs of abuse. That is why drugs take on so much salience, and everything else loses its importance.”